Build/Test Tools: Fix incorrect comment referencing wp-login.php.

The `#error` ID belongs to the database error container in
wp-includes/class-wpdb.php, not wp-login.php (which uses
`#login_error`). Update the comment to accurately describe
the defensive `.error:not(#error)` selector.

Author: josephfusco

tests/visual-regression/config/screenshot.css

@@ -52,8 +52,8 @@
 
 /*
  * Admin notices — various nags (PHP deprecation, updates, etc.).
- * `.error:not(#error)` excludes the `#error` container on wp-login.php,
- * which displays user-facing login errors and is part of the intended UI.
+ * `.error:not(#error)` excludes the `<div id="error">` database error
+ * container from wpdb (wp-includes/class-wpdb.php) as a defensive measure.
  */
 .notice,
 .update-nag,